One of the most versatile techniques used to study time resolved events on a pico- or femtosecond time scale is to use an ultrafast laser pulse to excite a material and then use a second pulse to measure the effect of the first pulse. By varying the delay between the first and second pulses a time resolved picture can be built up. This type of experiment is normally referred to as a pump-probe experiment. The technique is used in many applications such as carrier relaxation time measurements in semiconductors as described by D. G. McLEAN, M. G. ROE, A. I. D'SOUZA and P. E. WIGEN, Appl. Phys. Lett. 48 (1986) 992, electro-optic sampling as described by J. A. VALDMANIS and G. A. MOUROU, IEEE J. Quantum Electron. QE-22 (1986) 69, and electrical autocorrelation as described by T. F. CARRUTHERS and J. F. WELLER, Appl. Phys. Lett., 48 (1986) 460. The usual approach is to split the beam from a mainframe laser system, such as a mode locked solid state or dye laser, and send one half onto a mechanical delay line, such as a retro-reflector mounted on a stepper motor or a cam driven rocker arm as described by D. C. EDELSTEIN, R. B. ROMNEY and M. SCHEUERMANN, Rev. Sci. Instrum. 62 (1991) 579. A stepper motor will produce a scan range of several nanoseconds but will take some time to move over that range. In order to achieve an acceptable signal to noise ratio the time taken to acquire a set of data can be large. Rocker arm type systems will only allow for a scan range of a few hundred picoseconds or less, but will cover that range in a very short time and by using signal averaging techniques to improve the signal to noise ratio, the technique can acquire data sets in a comparatively short period of time.
Mechanical movement systems such as these are bulky and need careful alignment for good position sensitivity.